A radio frequency identification system (hereinafter referred to as an “RFID”), together with an interrogator and plural transponders (radio tags), makes up a radio communication system. In Japan, a standard for RFIDs has been established as a standard of ARIB (Association of Radio Industries and Businesses).
A configuration of an RFID system is shown in FIG. 8. A transponder 12 obtains a direct-current (DC) power supply (PS) required to drive internal circuits by rectifying at a detector circuit (DET) 14 a radio frequency power (radio waves) such as microwaves received from a transmitter circuit (Tx) 5 of an interrogator 1 via a transmitter antenna 3 and an antenna 13. At the same time, the transponder 12 extracts a clock signal (information) included in the radio frequency power and supplies both the clock signal and the DC power supply to a logic circuit (LOGIC) 15 and a memory circuit (MEM) 16 thereby letting the two circuits operate.
The logic circuit 15 having received the clock signal and started logic operation reads out relevant data from the memory circuit 16 and sends the data to the detector circuit 14. The detector circuit 14 using the data read out from the memory circuit 16 as a modulation signal subjects the radio frequency power received by the antenna 13 to modulation such as amplitude modulation. The modulated radio frequency power is radiated from the antenna 13 back to the interrogator 1.
The radio waves returned to the interrogator 1 are received by a receiver antenna 2 and are sent to a receiver circuit (Rx) 4 for data extraction. In this manner, data stored in the transponder 12 is transmitted to the interrogator 1.
There are cases in which the logic circuit 15 is caused to read out data from the memory circuit 16 when the logic circuit 15 reads a read command signal transmitted, along with the clock signal, as the information by the interrogator 1.
When data is written from the interrogator 1 to the memory circuit 16 of the transponder 12, a procedure similar to the above procedure is used. For the above write operation, in addition to the clock signal, a read/write command signal is also transmitted as the information to the transponder 12. Normally, in a system like this, the interrogator 1 is operated under the control of a controller (CONT) 6 connected via a communication path 7.
The interrogator 1 that supplies power to the transponder 12 using radio waves and also reads out data is permitted by the standard to radiate radio waves with a radio frequency power of 300 mW in a proximity remote coupling RFID system or with a radio frequency power of 10 mW in a close coupling or proximity coupling RFID system.
When a radio frequency output of as large as 300 mW is required in such an RFID system, the power consumption of the interrogator circuit as a whole reaches nearly 2 W. Hence, it need hardly be said that a large power supply is inevitably required even if the interrogator operation is limited to only when making communications. In the case of a battery-operated handy interrogator, a large-size battery will be required to secure such a large power supply capacity, making it difficult to reduce the size, weight and cost of an interrogator. In the case of a close coupling low-power interrogator, too, a battery power supply will be required inevitably making the interrogator heavy and expensive.